Two semifinalists are set in the "Champions of the 500" bracket challenge. Voting to determine the other half of the final four is under way now.

Sixteen legendary Indianapolis 500 winners were selected to square off in the digital voting platform running through May 23, leading to the historic 100th Running of the Indianapolis 500 presented by PennGrade Motor Oil on May 29. Fans can vote for their #Champsofthe500 once an hour while each pairing is open by visiting ims.com/champsofthe500. 

Fans also are encouraged to tout their votes by using the hashtag #Champsofthe500 in Facebook and Twitter posts. The voting contest runs in conjunction with Legends Day Honoring Champions of the 500 presented by Firestone at Indianapolis Motor Speedway on May 28.

In quarterfinal round voting that concluded earlier today, A.J. Foyt, the first four-time Indianapolis 500 winner and the driver with more all-time Indy car wins (67) than anyone, defeated two-time Indy 500 winner Bill Vukovich by a margin of 78 percent to 22 percent. In a matchup of three-time Indy 500 winners, Helio Castroneves (60 percent) defeated Bobby Unser (40 percent).

Quarterfinal pairings available for voting that began at noon ET today and runs through noon ET May 8 feature the other two four-time winners of the "Greatest Spectacle in Racing." Al Unser is pitted against three-time winner Johnny Rutherford while Rick Mears goes against 1969 Indy 500 champ Mario Andretti.

The four quarterfinal winners by popular vote advance to the semifinal round that begins May 9, with the championship round to follow. Show your Indianapolis 500 pride and vote for your #Champsofthe500.

#Champsofthe500 Quarterfinal Pairings:

May 2-5: A.J. Foyt def. Bill Vukovich; Helio Castroneves def. Bobby Unser

May 5-8: Al Unser vs. Johnny Rutherford; Rick Mears vs. Mario Andretti

Note: Voting begins and ends at noon ET on the designated days; drivers receiving the most votes in each pairing advance to the semifinal round to follow. Vote at ims.com/champsofthe500.